This invention relates to the polymerization and copolymerization of a mono-1-olefin, such as ethylene, with a higher alpha-olefin comonomer, such as 1-hexene.
This invention also relates to polyolefin compositions, methods for producing polyolefin compositions, and to processes for using polyolefin compositions for producing pipe.
This invention further relates to producing cost-effective ethylene polymers and copolymers of exceptionally broad molecular weight distributions and increased environmental stress crack resistance (ESCR) values, especially useful for high stiffness pipe resin.
Supported chromium catalysts long have been a dominant factor in the production of high density olefin polymers, such as polyethylene. As originally commercialized, these catalyst systems were used in solution polymerization processes. However, it became evident early, that a more economical route to many commercial grades of olefin polymers was a slurry process, that is, a polymerization process carried out at a low enough temperature that the resulting polymer is largely insoluble in the diluent.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing transition metals such as titanium, vanadium, chromium or other metals, either unsupported or on a support such as alumina, silica, aluminophosphate, titania, zirconia, magnesia and other refractory metals. Initially, such catalyst systems primarily were used to form homopolymers of ethylene. It soon developed, however, that comonomers such as propylene, 1-butene, 1-hexene, or other higher mono-1-olefins could be copolymerized with ethylene to provide resins tailored to specific end uses.
It is known in the art that when polymers of broad molecular weight distributions and improved physical properties such as environmental stress crack resistance (ESCR) or impact resistance are desired, chromium catalyst systems containing aluminophosphate supports can be employed. Polymers having a molecular weight distribution (Mw/Mn) of up to 30 can be obtained with aluminophosphate supported catalyst systems.
Aluminophosphate supports can be characterized by the amount of phosphate in the support, or more precisely, by the phosphorous to aluminum molar ratio (P/Al) of the composition. The P/Al molar ratio can vary from 0 for alumina (Al2O3), to 1 for stoichiometric aluminum phosphate (AlPO4). At a phosphorus to aluminum molar ratio of 1, a crystalline solid of very little surface area and minimal pore volume is obtained, so that the activity from such catalyst systems having a P/Al molar ratio of 1 is minimal. Chromium supported on alumina also provides very low activity. Therefore, in practice, the commercially preferred molar ratio of phosphorus to aluminum in chromium/aluminophosphate catalyst systems ranges from slightly more than 0 to slightly less than 1.
It is also known that the activity of aluminophosphate supported catalyst systems increases with the amount of phosphate in the support, reaching a maximum at around a P/Al molar ratio of 0.7 to 0.9. Below a P/Al molar ratio of 0.3, the activity is considered too low to be practical. Unfortunately, it is at these lower P/Al molar ratios that the broadest molecular weight distributions, and thus the highest ESCR and impact resistance values are obtained. Another disadvantage of chromium/aluminophosphate catalyst systems is that they incorporate comonomers, such as 1-hexene, very poorly. In fact, 1-hexene can kill the activity of chromium/aluminophosphate catalyst systems. Thus, while chromium/aluminophosphate catalyst systems are excellent for producing very high density blow molding resins, they do not function well for producing lower density copolymers, such as for film and pipe.